When transmitting data across a medium, wired or wireless, it is generally desirable to increase the amount of data that is sent at a given time to allow for a higher data rate for the underlying system. In a single-antenna, wireless transmission system, typically a single stream of data is transmitted.
However, using a multiple antenna system, multiple streams can be transmitted and received using the same time and frequency resources, and correspondingly more information can be sent over the available bandwidth. As shown below, it is possible to use multiple antenna systems and multi-input, multi-output (MIMO) systems interchangeably. In MIMO systems, there are a number of possible signal paths through the transmission medium equal to the number of transmitting antennas multiplied by the number of receiving antennas.
FIG. 1 is a block diagram illustrating a conventional MIMO transmitter/receiver pair. As shown in FIG. 1, a transmitter includes a MIMO modulator 110, an antenna 130, and an antenna 135, while a receiver includes a MIMO demodulator 120, an antenna 140, and an antenna 145. The MIMO modulator receives transmit signals and modulates them for transmission over antenna 130 and antenna 135. These signals are received at antenna 140 and antenna 145, and the received signals are demodulated at the MIMO demodulator 120 to extract a received signal.
Because there are two transmitting antennas 130 and 135, and two receiving antennas 140 and 145, there are four possible signal paths through the transmission channel. There is a path from the antenna 130 to the antenna 140; a path from the antenna 130 to the antenna 145; a path from the antenna 135 to the antenna 140; and a path from the antenna 135 to the antenna 145. MIMO systems with a larger number of transmitting or receiving antennas would have a correspondingly larger number of possible signal paths.
The MIMO receiver antennas 140 and 145 thus each receive a copy of each signal transmitted from all of the transmitting antennas. But each copy of a transmitted signal from a given transmitter antenna passes through a different portion of the transmission channel, and is multiplied by a different channel coefficient.
In particular, h11 represents a channel portion from a transmitter 130 antenna to a receiver antenna 140; h21 represents a channel portion from the transmitter antenna 130 to a receiver antenna 145; h12 represents a channel portion from the transmitter antenna 135 to the receiver antenna 140; and h22 represents a channel portion from the transmitter antenna 135 to the receiver antenna 145. A general representation H of the transmission channel, as it pertains to the MIMO transmitter and receiver, can then be written as a matrix of the individual channel coefficients h11, h12, h21, and h22. In other words,
                              H          22                =                              [                                                                                h                    11                                                                                        h                    12                                                                                                                    h                    21                                                                                        h                    22                                                                        ]                    .                                    (        1        )            
More generically a channel HMN can be written as an (N×M) matrix where M is the number of transmit antennas and N is the number of receiver antennas. Thus, a (3×2) MIMO system will have a channel represented by a channel matrix H32:
                                          H            32                    =                      [                                                                                h                    11                                                                                        h                    12                                                                                        h                    13                                                                                                                    h                    21                                                                                        h                    22                                                                                        h                    23                                                                        ]                          ,                            (        2        )            
while a (4×2) MIMO system will have a channel represented by a channel matrix H42:
                              H          42                =                  [                                                                      h                  11                                                                              h                  12                                                                              h                  13                                                                              h                  14                                                                                                      h                  21                                                                              h                  22                                                                              h                  23                                                                              h                  24                                                              ]                                    (        3        )            
A transmitter with a single antenna can only modulate data in time. In other words, it can only send different portions of data through the antenna at different times. A MIMO transmitter, in contrast, can modulate the data in both space and time. In other words, in addition to sending different portions of data at different times, the MIMO transmitter can also send different portions of data over the different antennas as the same time. This allows a MIMO transmitter to send a larger amount of data in the same time period as compared to a single antenna system.
A MIMO transmitter typically maps a stream of modulated data bits to be sent to a receiver into an appropriate space time code (STC) for transmission. This space-time code (STC) uses the dimensions of space and time to encode the multiple data bits. Space is employed through the use of an array of multiple antennas; time is used by having each antenna in the array send a series of transmitted symbols in succession. In particular, one or more bits from a transmit bit stream are mapped onto a plurality of symbols s1-sN chosen from a known constellation of available symbol values, which are used to populate an STC matrix S that represents how the symbols will be sent to the receiver. In particular each symbol will be chosen to represent one or more bits from the bit stream.
The STC matrix S is an (M×K) matrix, where M represents the number of antennas used by the transmitter, and K represents the number of symbols sent by each transmitter for a given set of mapped bits. By way of example a MIMO system in which the transmitter had four antennas (i.e., M=4), and for which each antenna transmits two symbols for a given set of mapped bits (i.e., K=2) will be described. It will be understood, however, that this description can apply generally to situations in which M and K have different values.
In this exemplary (4×L) MIMO system, where L>=2, the STC matrix S is a (4×2) matrix that can be written as:
                              S          =                                    S              42                        =                          [                                                                                          s                      1                                                                                                  s                      5                                                                                                                                  s                      2                                                                                                  s                      6                                                                                                                                  s                      3                                                                                                  s                      7                                                                                                                                  s                      4                                                                                                  s                      8                                                                                  ]                                      ,                            (        4        )            
Where s1, s2, s3, s4, s5, s6, s7, and s8 represent the symbols that the transmitter will be sending to the receiver.
The MIMO receiver receives the various signals at its receiver antennas from across the various individual channel portions, and its MIMO demodulator 120 uses MIMO signal processing to extract the different portions of data that were sent.
In addition, rather than increasing data rate, a MIMO system could also use be used to increase the range of the transmission system by increasing the level of redundancy, error correction, or the like.
One way to estimate the transmitted symbols is to use a minimum mean square estimation (MMSE) operation. MMSE receivers operate by minimizing the mean square estimate of an error in the received signal in an effort to optimize data reception, as would be understood by one skilled in the art.
With MMSE, the receiver measures the error between a set of estimated parameters and the actual value of those parameters. By squaring this error term and minimizing it, the actual error can be minimized MMSE estimation techniques are well known in the art.
Still another way to improve error performance is to use an MMSE operation with successive interference cancellation (SIC). In an MMSE-SIC system, a plurality of co-located signals are iteratively cancelled out.
MMSE-SIC uses an MMSE matrix filter at each stage to estimate received symbol (i.e., symbols in an embodiment in which each received signal contains one symbol). In order to cancel out the received symbol, the effect of that symbol is removed from the received signal and a new resultant reduced system of equations is formed. MMSE-SIC techniques are well know in the art, as are the methods of cancelling the effect of given symbols and deriving the equations of the resultant system for the rest of the symbols.
A typical MMSE matrix filter is defined by the following equation:
                              G          MMSE                =                                            (                                                                    H                    H                                    ⁢                  H                                +                                                      1                    ρ                                    ⁢                  I                                            )                                      -              1                                ⁢                                    H              H                        .                                              (        5        )            
where GMMSE is the gain of the filter, H is the channel matrix, HH/is the Hermitian conjugate of the channel matrix H, I is the identity matrix, and ρ is the averaged SNR, calculated by the equation:
                              ρ          =                                    E              s                                      N              o                                      ,                            (        6        )            
where Es is the energy per symbol, and No is the noise power spectral density.
However, because a wireless channel H is a fading channel (i.e., signals passing through it reflect off of objects and take multiple paths to the receivers), each transmitted signal experiences fading, causing a reconstructed signal at the MIMO receiver to itself experience fading. This fading can require an increased cost and complexity for the MIMO receiver to properly process the faded signal, and increased time for such back end processing.